UV-embossed textured back reflector structures for thin film silicon solar cells
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1245-A07-04
UV-embossed textured back reflector structures for thin film silicon solar cells Jordi Escarré1, Karin Söderström1, Oscar Cubero1, Franz-Josef Haug1, and Christophe Ballif1 1 Ecole Polytechnique Fédérale de Lausanne (EPFL), Institut of Microengineering (IMT), Photovoltaics and Thin Film Electronics Laboratory, 2000 Neuchâtel, Switzerland ABSTRACT In this work, we study the replication of nanotextures used in thin film silicon solar cells to enhance light trapping onto inexpensive substrates such as glass or polyethylene naphtalate (PEN). Morphological analysis was carried out to asses the quality of these replicas. Moreover, single and tandem a-Si:H solar cells were deposited on top of the master and replica structures to verify their suitability to be used as substrates for solar cells in n-i-p configuration. We find stabilized efficiencies around 8 % which are similar for tandem cells on masters and PEN replicas. INTRODUCTION The use of plastic foils as substrates for thin film amorphous silicon solar cells is an interesting approach to reduce manufacturing costs by means of roll to roll deposition [1]. In these devices, optical confinement techniques play a crucial role in order to reduce the thickness of the intrinsic layer, leading to solar cells with better carrier collection and higher stable efficiencies. The light trapping is achieved by texturing some of the layers or the substrate itself. In n-i-p configuration, the texture is usually obtained on the back reflector by growing rough metallic [2] or transparent conducting oxides layers [3] at high temperatures. These temperatures make plastics as PET or PEN not suitable to be used as substrates. In order to avoid substrate limitations, in this contribution we study the controlled transfer of a roughness on to another substrate by means of an “in house” ultra violet nano imprinting lithography (UV-NIL) system, a technology that can easily be scaled up to large areas and roll to roll processing [4]. As grown textured boron doped zinc oxide deposited by low-pressure chemical vapor deposition (LPCVDZnO) and as grown silver deposited at high temperature (~ 400 °C) by sputtering were used as masters in the replication process. The replicas were obtained on glass and PEN. The quality of the replication was evaluated by a morphological comparision of both, the master and the replica surfaces. Moreover, single and tandem a-Si:H solar cells were deposited on masters and replicas, showing comparable performance. EXPERIMENT Glass (0.5 mm thick, Schott AF45) and polyethylene-naphtalate (50 µm thick PEN) with a size of 4×4 cm2 were used as substrates in the replication process. An “in house” [5] UV-NIL system was utilized in this work to transfer nanometric random textures onto the substrates. The original texture (Master) is molded in polydimethylsiloxane (PDMS, Sylgard 184, Dow Corning). The PDMS is prepared with a ratio 10:1 between the base and the curing agent, degassed in vacuum, dispensed onto the master, and cured at 46°C for 12 hours. After separati
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